Method for producing indium-containing aqueous solution

ABSTRACT

As an effective method for reusing ITO sintered bodies, there is provided a method for producing an aqueous solution containing indium ions which comprises a step of subjecting an acidic solution containing indium ions and tin ions to an electrolytic treatment to precipitate metallic tin and a step of removing or re-dissolving the precipitated metallic tin.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing an aqueous solution containing indium ions.

2. Description of Related Art

An aqueous solution containing indium ions is used for producing an indium tin oxide (hereinafter sometimes referred to as “ITO”) sintered body. An ITO sintered body is used as a target for producing an ITO thin film by a sputtering method. An ITO thin film is utilized as a transparent electric conductive film for a liquid crystal display because of its high electric conductivity and excellent transparency.

When an ITO sintered body is used as a target in a sputtering method, the ITO sintered body is eroded with progress of sputtering. The erosion proceeds not uniformly, but locally. When the erosion progresses, penetrated holes are formed through the ITO sintered body, and hence the ITO sintered body is exchanged for a new one before the formation of holes. ITO sintered bodies which have some still usable portions are not abandoned as they are, but are reused, and various methods are proposed for the reuse of ITO sintered bodies.

These proposals all employ complicated steps in an attempt to recover high purity metallic indium or indium oxide.

For example, JP-A-2000-169991 discloses a method for electrolytic extraction of metallic indium having a purity of 99.9999% or higher after precipitating and removing impurities as hydroxides and sulfides.

JP-A-8-91838 discloses a method for recovery of an indium oxide powder of 99.99% in purity by repeating extraction and back extraction according to a solvent extraction method.

JP-A-10-204673 discloses a method for recovery of metallic indium of 99.99% in purity by carrying out dissolving treatment of an ITO target, replacement precipitation of impurity, and subsequent electrolytic extraction of the metallic indium.

The recovered metallic indium of high purity is reused as a raw material for preparation of an indium oxide powder. Indium oxide prepared in this way or recovered as mentioned above is mixed with tin oxide at a given ratio, and the mixture is press molded and sintered to obtain an ITO sintered body (sintered body producing method 1 disclosed in JP-A-3-207858).

Metallic indium or indium oxide recovered by any of these methods is very high in purity and hence complicated steps are required and production cost is high. Furthermore, tin in the ITO sintered body is not reused.

As a method of producing an ITO sintered body, there is a method which comprises co-precipitating indium hydroxide and tin hydroxide, recovering the resulting particles, molding the particles and sintering the molded particles (sintered body producing method 2 disclosed in JP-A-7-247162). According to the method disclosed in the patent document, In(NO₃)₃ and SnCl₄.5H₂O were used as the material for the co-precipitation of indium hydroxide and tin hydroxide, and the tin ion was tetravalent and the co-precipitated particles were In(OH)₃—Sn(OH)₄.

BRIEF SUMMARY OF THE INVENTION

The inventors have conducted a research on a more effective method for reuse of ITO sintered bodies and accomplished the present invention.

That is, the present invention relates to a method for producing an aqueous solution containing indium ions which comprises a step of subjecting an acidic solution containing indium ions and tin ions to an electrolytic treatment to precipitate metallic tin and a step of removing or redissolving the precipitated metallic tin. The method of the present invention includes an electrolytic treatment as a main step, being different from conventional methods.

According to the method of the present invention in which metallic tin is removed, being different from the conventional methods of reuse including complicated steps, indium is not recovered as metallic indium or indium oxide of extremely high purity, but is recovered by a simple electrolytic treatment in a purity of such an extent as capable of being effectively utilized at the subsequent steps for producing ITO sintered body. The method is very simple, requires low production cost and is industrially advantageous.

Furthermore, according to the method of the present invention in which metallic tin is re-dissolved, tin in the ITO sintered body is also reused. Moreover, there is obtained an aqueous solution which contains indium ions and is high in the content of divalent tin ions, and when this aqueous solution is used, an ITO sintered body of high density can be obtained, which is suitable for producing an ITO thin film having a high electric conductivity. In addition, the compositional ratio of indium and tin in the original sintered body is nearly reproduced, and thus the method is industrially advantageous.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for producing an aqueous solution containing indium ions which comprises a step of subjecting an acidic solution containing indium ions and tin ions to an electrolytic treatment to precipitate metallic tin and a step of removing or re-dissolving the precipitated metallic tin.

The First Embodiment

In a preferred embodiment of the present invention, the current quantity for the electrolytic treatment is such one as capable of substantially precipitating tin ions from the acidic solution by converting tin ions to metallic tin. The precipitated metallic tin is removed.

Being different from conventional methods, this method does not recover indium as metallic indium or indium oxide of extremely high purity, but recovers indium by a simple electrolytic treatment in a purity of such an extent as capable of being effectively utilized at the subsequent steps for producing ITO sintered body. The method is very simple, requires low production cost and is industrially advantageous.

The current quantity for the electrolytic treatment is such one with which the tin ions can be substantially precipitated from the acidic solution by converting tin ions to metallic tin, and is preferably not less than 1-fold and not more than 3-fold, more preferably not less than 1-fold and not more than 2-fold of the current quantity actually needed for converting 80% of tin ions contained in the acidic solution to metallic tin. If the current quantity is too much, metallic indium is precipitated, resulting in reduction of indium ion concentration. The precipitated tin which deposits on an electrode upon the completion of the electrolytic treatment can be separated together with the electrode and the precipitated tin which is liberated from the electrode can be separated by filtration or the like. When the electrolytic treatment is stopped, dissolution of the precipitated metallic tin spontaneously begins and hence it is preferred to carry out the removal of the electrode or the filtration immediately after completion of the electrolytic treatment.

The current density in the electrolytic treatment is preferably not less than 50 Am⁻² and not more than 800 Am⁻², more preferably not less than 100 Am⁻² and not more than 700 Am⁻², further preferably not less than 400 Am⁻² and not more than 600 Am⁻². If the current density exceeds 800 Am⁻², metallic indium tends to be also precipitated in addition to the metallic tin. If the current density is less than 50 Am⁻², a long time may be required for the precipitation of metallic tin.

The amount of tin ions remaining in the aqueous solution is preferably 2% by weight or less, more preferably 1% by weight or less based on the total amount of indium ions and tin ions.

The thus obtained aqueous solution containing indium from which tin ions are substantially removed may be utilized in the following manner. That is, metallic indium may be precipitated by further electrolyzing the aqueous solution. Furthermore, an indium oxide powder may be produced by oxidizing the metallic indium, and an ITO sintered body may be produced by mixing the powder with a tin oxide powder, molding the mixture and sintering the molded mixture.

Moreover, an aqueous solution containing divalent tin ions is added to the indium-containing aqueous solution from which tin ions are substantially removed, thereby to prepare an aqueous solution containing indium ions and mainly divalent tin ions, and an ITO sintered body may be produced from the aqueous solution as mentioned hereinafter.

In any of these methods, the amount of the tin oxide powder added and the amount of the tin ions added can be determined in relation with the concentration of indium ions depending on the amount of tin contained in the final ITO powder. The amount of tin contained in the ITO powder in terms of oxide is 2-20% by weight, usually 10-20% by weight based on the total amount of indium oxide and tin oxide.

The Second Embodiment

In another preferred embodiment of the present invention, the electrolytic precipitated metallic tin is redissolved.

In an aqueous solution, tin is present in the state of being divalent or tetravalent. According to the present invention, tetravalent tin ions are precipitated as metallic tin in preference to divalent tin ions by the electrolytic treatment, and the precipitated metallic tin redissolves in the aqueous solution as divalent tin ions. Therefore, the amount of divalent tin ions with respect to the total tin ions in the aqueous solution can be increased without substantial increase of tin ions.

The inventors have found that when precipitates of hydroxides of indium and tin are produced from an aqueous solution containing indium ions and tin ions and containing divalent tin ions in an amount of 50% by weight or more of the total tin ions (total of divalent tin ions and tetravalent tin ions), an ITO sintered body obtained therefrom has a high density, and when this sintered body is used as a sputtering target, an ITO thin film having a high electric conductivity can be obtained (Japanese Patent Application No. 2003-319439). In the above invention, the inventors have proposed a method for producing an aqueous solution containing divalent tin ions in an amount of 50% by weight or more of the total tin ions by adding metallic tin to an acidic solution containing mainly tetravalent tin ions which is obtained by dissolving the ITO sintered body in an acid, thereby decreasing tetravalent tin ions and increasing divalent tin ions in accordance with the following formula (1): Sn+Sn⁴⁺→2Sn²⁺  (1) According to this method, the amount of the total tin ions in the aqueous acidic solution increases and the amount of tin ions with respect to indium ions increases.

On the other hand, in the method of the present invention, it is utilized that tetravalent tin ions are precipitated as metallic tin by the electrolytic treatment in preference to divalent tin ions, and the precipitated metallic tin is redissolved as divalent tin ions. Therefore, an aqueous solution containing divalent tin ions in an amount of 50% by weight or more of the total tin ions can be simply produced without substantial increase of tin ions.

The indium-containing aqueous solution obtained by this method is low in the content of tetravalent tin ions, and when substantially all of the precipitated tin is dissolved, the solution has tin ion and indium ion at nearly the same compositional ratio as in the original sintered body. An ITO sintered body may be produced by a known method by co-precipitating these ions and calcining the precipitate and molding the calcined powder and sintering. The ITO sintered body produced using the indium-containing aqueous solution obtained by the method of the present invention has a high density and is suitable as a sputtering target for producing a transparent electric conductive film. Moreover, the present invention is also industrially advantageous in view of the facts that tin in the ITO sintered body is also reused and nearly the same compositional ratio of indium and tin as in the original sintered body can be reproduced.

It is preferred that the acidic solution used contains indium ions and tin ions and the content of tetravalent tin ions exceeds 50% by weight in the total tin ions. For example, tin ions contained in an acidic solution obtained by dissolving an ITO sintered body in an acid are mainly tetravalent, and such an acidic solution can be suitably used.

The current quantity passed in the electrolytic treatment is preferably half or more of the current quantity required for precipitating tetravalent tin ions as metallic tin so that the content of divalent tin ions reaches 50% by weight or more of the total amount of tin ions. Since at least a part, preferably, substantially all of the precipitated metallic tin is redissolved as divalent tin ions, the current quantity is half or more of the current quantity required for precipitating tetravalent tin ions as metallic tin. Specifically, first the amount of tetravalent tin ions necessary for decreasing the proportion of tetravalent tin ions in the total tin ions to less than 50% by weight in the acidic solution is calculated, and then a current quantity necessary to reduce the tetravalent tin ions to metallic tin is calculated. The current quantity passed in the electrolytic treatment is not less than 0.5-fold, preferably not less than 0.5-fold and not more than 10-fold, more preferably not less than 0.5-fold and not more than 5-fold of the current quantity obtained above. If the current quantity is too much, metallic indium is precipitated subsequent to metallic tin. When the electrolytic treatment is discontinued, all of the precipitated metallic indium may be dissolved in the acidic solution, and, therefore, after completion of dissolution of all the metallic indium, the indium ion concentration in the acidic solution does not change from the concentration before the electrolytic treatment, but energy efficiency lowers.

The current density in the electrolytic treatment is preferably not less than 50 Am⁻² and not more than 2000 Am⁻², more preferably not less than 100 Am⁻² and not more than 1500 Am⁻², further preferably not less than 400 Am⁻² and not more than 600 Am⁻². If the current density exceeds 2000 Am⁻², hydrogen gas is sometimes generated from cathode. If it is less than 50 Am⁻², a long time is required for the precipitation of metallic tin.

At the stage of completion of the electrolytic treatment, the proportion of divalent tin ions in the total tin ions in the acidic solution sometimes already reaches 50% by weight or more. Furthermore, all of the metallic tin precipitated at the stage of completion of the electrolytic treatment may be present on the cathode or may be present in the state of being partly liberated in the solution.

Then, the precipitated metallic tin and, if present, the liberated tin are dissolved and tetravalent tin ions remaining in the acidic solution is reduced to divalent tin ions in accordance with the formula (1). The dissolution of metallic tin spontaneously begins immediately after discontinuation of the electrolytic treatment. Sn+Sn⁴⁺→2Sn²⁺  (1)

The precipitated metallic tin may be separated from the acidic solution without using it and metallic tin separately prepared may be added to the acidic solution and dissolved therein, but it is preferred to use the precipitated metallic tin because the amount of tin ions in the acidic solution does not change from the amount before the electrolytic treatment.

The reduction treatment with dissolution of metallic tin is preferably carried out in an inert gas atmosphere such as N₂ or Ar, more preferably carried out in an inert gas atmosphere while stirring the acidic solution.

The temperature at the time of dissolution of the metallic tin is not particularly limited and may be optionally selected within the industrially usually employed range. The dissolution is usually carried out at a temperature of not lower than 0° C. and not higher than 90° C., and the higher temperature is preferred since the reduction of tetravalent tin ions by the dissolution of the metallic tin is accelerated. The time of the reduction treatment by the dissolution of the metallic tin is not necessarily limited since it varies depending on the reaction temperature or the proportion of divalent tin ions in the aqueous solution to be finally obtained, and it is usually 1 hour or more, preferably 3 hours or more, more preferably 5 hours or more.

In case a part of metallic tin remains in undissolved state after the reduction treatment by the dissolution of metallic tin, such metallic tin may be separated from the aqueous solution by a method such as filtration.

Furthermore, only one electrolytic operation and one operation of dissolution of metallic tin are described above, but also, for example, the electrolytic operation and the operation of dissolution of metallic tin may-be repeatedly carried out several times.

The proportion of divalent tin ions in total tin ions in the aqueous solution obtained as mentioned above is 50% by weight or more, preferably 70% by weight or more, further preferably 80% by weight or more.

If necessary, concentrations of indium and tin may be adjusted by further adding an aqueous solution containing divalent tin ions or an aqueous solution containing indium ions. In this case, the concentrations of indium and tin may be determined in relation with the concentration of indium depending on the amount of tin contained in the ITO powder to be obtained finally. The amount of tin contained in the ITO powder is 2-20% by weight, usually 10-20% by weight in terms of an oxide based on the total amount of indium oxide and tin oxide.

The resulting aqueous solution containing indium and divalent tin ions is easily oxidized, for example, by leaving it in the air, and the divalent tin ions change to tetravalent tin ions. Therefore, the aqueous solution is preferably stored in an inert atmosphere in a container sealed hermetically.

When electrolysis is started, metallic tin is usually deposited on a cathode electrode. However, if the current density is low and the temperature of the acidic solution is high, it sometimes occurs that the precipitation of metallic tin cannot visually be observed and the precipitated metallic tin is immediately dissolved. Therefore, the above method includes the case where the precipitation of metallic tin is not visually observed. Further, precipitation of metallic tin and dissolution of metallic tin sometimes proceed simultaneously, and the present invention also includes this case.

Electrolytic Treatment

Conditions of electrolytic treatment and others which are common to the above embodiments 1 and 2 are as follows.

The method of electrolytic treatment may be either of constant current method and constant voltage method (constant potential method), but the constant current method is preferred. In the case of using the constant current method, the electrolytic treatment is carried out with the current density as mentioned above.

In the case of using the constant voltage method, the cathode potential is −0.6 V or higher, preferably −0.45 V or higher, more preferably −0.4 V or higher according to standard hydrogen electrode. If the potential is lower than −0.6 V, hydrogen gas may be generated from the cathode electrode plate.

Materials of the electrode used for the electrolytic treatment are preferably insoluble platinum, indium, dimensionally stable electrode or carbon electrode plate as an anode electrode, and tin, copper, titanium or platinum as a cathode electrode. When the electrolysis is started, metallic tin is deposited on a cathode electrode.

The temperature of the electrolyte during electrolytic treatment is not particularly limited and may be optionally selected within the range which is usually employed for industrial electrolytic treatment. The electrolytic treatment is usually carried out at a temperature of 0° C. or higher and 80° C. or lower.

Since the divalent tin ions are gradually oxidized to tetravalent tin ions with oxygen in the air, the electrolytic treatment is preferably carried out in an inert gas atmosphere such as N₂ or Ar. More preferably, the electrolytic treatment is carried out in an inert gas atmosphere while stirring the acidic solution.

When hydrochloric acid is contained in the acidic solution, chlorine gas may be generated from the anode electrode. In this case, it is preferred to partition the electrolytic cell with a cation exchange membrane and to carry out the electrolytic treatment by passing a current through an aqueous sulfuric acid solution in the anode chamber and the acidic solution containing indium ions and tin ions in the cathode chamber.

When dissolution of metallic indium as the anode electrode is allowed to preferentially take place, generation of chlorine gas can sometimes be inhibited even hydrochloric acid is contained in the acidic solution. In this case, the electrolytic treatment may be carried out without using the cation exchange membrane.

As the acidic solution containing indium ions and tin ions, mention may be made of, for example, an acidic solution prepared by dissolving a compound containing indium, tin and oxygen, such as ITO or an indium hydroxide-tin hydroxide mixture in an acid such as hydrochloric acid, sulfuric acid or nitric acid. The acidic solution is not limited to the above examples. Industrially, the present invention is effectively employed for reusing ITO sintered bodies removed from used ITO targets, ITO powders below quality standards, ITO sintered bodies below quality standards, cutting wastes of ITO sintered bodies, etc.

A method using a used ITO target will be explained below.

Since a used ITO target is recovered in such a state that an ITO sintered body is adhered to a backing plate made of copper by indium soldering and the like, the recovered target is heated to about 150-200° C. and the ITO sintered body is peeled off from the backing plate. In some case, the indium solder used for adhering the ITO sintered body to the backing plate may attach to the ITO sintered body peeled off and removed. The indium solder sometimes contains impurities such as Cu and Pb, and, besides, extraneous substances containing Si, Al, Fe, etc. sometimes attach to the surface of the ITO sintered body. Therefore, it is preferred to wash the ITO sintered body with an acid to remove the indium solder or extraneous substances.

It is preferred that the ITO sintered body is previously ground for improving dissolution rate into acid. The grinding method is not particularly limited, and there can be used a jaw crusher, roll crusher, disk mill, vibration mill and the like which are industrially usually employed. The material of a part of these grinding machines which contacts with the ITO sintered body to be ground is preferably ceramics such as alumina, zirconia or tungsten carbide which is hardly worn. In the case of the above material being metal, the metal sticks to the ground ITO sintered body to cause contamination, and it sometimes become necessary to remove metal impurities from a solution prepared by dissolving the ITO sintered body (hereinafter sometimes referred to as merely “solution”). The size of the ITO sintered body after grinding is preferably 20 mm or less, more preferably 2 mm or less, most preferably 0.5 mm or less.

The acid for dissolving the ITO sintered body includes, for example, hydrochloric acid, sulfuric acid, nitric acid or the like. Hydrochloric acid which is high in dissolution rate of ITO is preferred. A case of using hydrochloric acid will be explained below. The dissolution method is not particularly limited, and mention may be made of, for example, a method of charging hydrochloric acid and the ground ITO sintered body in a reaction vessel, followed by stirring.

The temperature and time for dissolving the ITO sintered body with acid are not particularly limited, and industrially advantageous temperature and time can be selected. The temperature is usually 40° C. or higher and 100° C. or lower, preferably 60° C. or higher and 80° C. or lower, and the dissolution time is usually 100 hours or less, preferably 50 hours or less, further preferably 24 hours or less. The indium concentration in the solution obtained by dissolving the ITO sintered body is preferably not less than 50 g/L and not more than 350 g/L, more preferably not less than 100 g/L and not more than 350 g/L.

In the resulting solution, there sometimes remain fragments of undissolved ITO sintered body or ceramics particles incorporated from the parts of the grinding machines. In this case, these solids are removed by a solid-liquid separation such as filtration and only the liquid is recovered.

Furthermore, Zr, Al, Si, Fe and the like are sometimes contained as impurities. In this case, it is preferred to add a step of allowing the solution to contact with an ion exchange resin such as a cation exchange resin or an anion exchange resin to remove these impurities. Especially, when a solution obtained by grinding an ITO sintered body and dissolving the ground ITO sintered body is used, there is a high possibility of containing Zr, Al, Si, Fe and the like as impurities, and it is preferred to add a step of allowing the solution to contact with an ion exchange resin to remove these impurities. The tin ions contained in the thus obtained ITO solution are tetravalent, and there are contained ion species similar to those in an aqueous solution obtained by dissolving indium chloride and stannic chloride. The above step may be carried out before or after the electrolytic treatment, and may be carried out after the reduction treatment.

Having thus generally described the present invention, the following specific examples are provided to illustrate the invention. The examples are not intended to limit the scope of the invention in any way.

EXAMPLES

As the acidic solution containing indium ions and tin ions, the following solutions were used in Examples 1-4.

[Acidic solution A]: A used ITO target was ground to about 1-4 mm by a jaw crusher and dissolved in hydrochloric acid having a concentration of 35% by weight. The resulting solution was diluted with pure water to obtain an acidic solution A having an In concentration of 167.2 g/L and an Sn concentration of 17.7 g/L.

[Acidic solution B]: A used ITO target was ground to about 1-4 mm by a jaw crusher and dissolved in hydrochloric acid having a concentration of 35% by weight. The resulting solution was diluted with pure water and passed through a column packed with a cation exchange resin to remove impurity Zr, thereby obtaining an acidic solution B having an In concentration of 159.6 g/L and an Sn concentration of 14.8 g/L.

The indium and tin ion concentrations were measured by ICP spectrometry.

Example 1

In a 100 mL beaker was charged 98 mL of the acidic solution A, followed by stirring while blowing N₂ gas into the beaker. An electrolytic treatment was carried out using a metallic indium plate of 30×20 mm as an anode electrode and a metallic tin plate of 30×30 mm as a cathode electrode at a current density of 780 Am⁻² at room temperature (about 25° C.) for 60 minutes. Concentrations of indium ions and tin ions in the resulting aqueous solution were 182.9 g/L (indium ion concentration increased over the concentration before the electrolytic treatment since there occurred dissolution of metallic indium used as the anode) and 2.5 g/L respectively. The tin ion concentration after the electrolytic treatment was 14% of the tin ion concentration before the electrolytic treatment (1.3% by weight of the total metal ions).

Example 2

50 mL of the acidic solution B was charged in a cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring while blowing N₂ gas thereinto. Furthermore, sulfuric acid of 1 N in concentration was charged in an anode electrode chamber. An electrolytic treatment was carried out using a metallic copper plate of 36×25 mm as a cathode electrode and a platinum plate of 30×20 mm as an anode electrode at a current density of 200 Am⁻² at room temperature (about 25° C.) for 3 hours. Concentrations of indium ions and tin ions in the resulting aqueous solution were 158.7 g/L and 2.8 g/L respectively. The tin ion concentration after the electrolytic treatment was 19% of the tin ion concentration before the electrolytic treatment (1.7% by weight of the total metal ions).

Example 3

50 mL of the acidic solution B was charged in a cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring while blowing N₂ gas thereinto. Furthermore, sulfuric acid of 1 N in concentration was charged in an anode electrode chamber. An electrolytic treatment was carried out using a Cu plate of 36×25 mm as a cathode electrode and a platinum plate of 30×20 mm as an anode electrode at a current density of 400 Am⁻² at room temperature (about 25° C.) for 2 hours. The indium ion concentration and tin ion concentration in the resulting aqueous solution were 155.6 g/L and 1.5 g/L respectively. The tin ion concentration after the electrolytic treatment was 10% of the tin ion concentration before the electrolytic treatment (0.95% by weight of the total metal ions).

Example 4

50 mL of the acidic solution B was charged in a cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring while blowing N₂ gas thereinto. Furthermore, 1 N sulfuric acid was charged in an anode electrode chamber. An electrolytic treatment was carried out using a metallic copper plate of 36×25 mm as a cathode electrode and a platinum plate of 30×20 mm as an anode electrode at a current density of 600 Am⁻² at room temperature (about 25° C.) for 2 hours. The indium ion concentration and tin ion concentration in the resulting aqueous solution were 145.8 g/L and 0.3 g/L respectively. The tin ion concentration after the electrolytic treatment was 2% of the tin ion concentration before the electrolytic treatment (0.2% by weight of the total metal ions).

As the acidic solution containing indium ions and tetravalent tin ions, the following solutions were used in Examples 5-8. The solutions substantially do not contain divalent tin ion.

[Acidic solution B]: A used ITO target was ground to about 1-4 mm by a jaw crusher and dissolved in an aqueous hydrochloric acid solution having a concentration of 35% by weight. The resulting aqueous solution was diluted with pure water and allowed to contact with a cation exchange resin to remove the impurity Zr, thereby obtaining an acidic solution A having an indium ion concentration of 159.6 g/L and a tetravalent tin ion concentration of 14.8 g/L.

[Acidic solution C]: Stannic chloride pentahydrate (SnCl₄.5H₂O) was added to an acidic solution obtained by dissolving metallic indium in an aqueous hydrochloric acid solution having a concentration of 35% by weight to prepare an acidic solution B containing indium ions and tetravalent tin ions and having an indium ion concentration of 334.7 g/L and a tetravalent tin ion concentration of 36.3 g/L.

[Acidic solution D]: Stannic chloride pentahydrate (SnCl₄.5H₂O) was added to an acidic solution obtained by dissolving metallic indium in an aqueous hydrochloric acid solution having a concentration of 35% by weight to prepare an acidic solution C containing indium ions and tetravalent tin ions and having an indium ion concentration of 346.2 g/L and a tetravalent tin ion concentration of 36.5 g/L.

[Acidic solution E]: A used ITO target was ground to about 1-4 mm by a jaw crusher and dissolved in an aqueous hydrochloric acid solution having a concentration of 35% by weight to prepare an acidic solution D containing indium ions and tetravalent tin ions and having an indium ion concentration of 333.4 g/L and a tetravalent tin ion concentration of 34.3 g/L.

The indium ion concentration in the acidic solution was measured by ICP spectrometry, the total tin ion concentration (concentration of divalent tin ions+tetravalent tin ions) was measured by iodometric titration or ICP spectrometry, and the divalent tin ion concentration was measured by iodometric titration.

Example 5

50 mL of the acidic solution B was charged in the cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring while blowing N₂ gas into the acidic solution. Furthermore, 50 mL of 1 N aqueous sulfuric acid solution was charged in the anode electrode chamber. An electrolytic treatment was carried out using a copper plate of 36×25 mm as a cathode electrode and a platinum plate of 30×20 mm as an anode electrode at a current density of 400 Am⁻² at room temperature (about 25° C.) for 55 minutes. Then, the electrolytic treatment was stopped and the content was left to stand for 5 hours to obtain an acidic solution having an indium ion concentration of 157 g/L, a total tin ion concentration of 10.5 g/L and a divalent tin ion concentration of 6.4 g/L. The proportion of divalent tin ions in the tin ions in this acidic solution was 61% by weight.

Example 6

50 mL of the acidic solution C was charged in the cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring while blowing N₂ gas thereinto. Furthermore, 50 mL of 1 N aqueous sulfuric acid solution was charged in the anode electrode chamber. An electrolytic treatment was carried out using a copper plate of 36×25 mm as a cathode electrode and a platinum plate of 30×20 mm as an anode electrode at a current density of 600 Am⁻² at room temperature (about 25° C.) for 55 minutes. Then, the electrolytic treatment was stopped and the content was left to stand for 5 hours to obtain an acidic solution having an indium ion concentration of 331 g/L, a total tin ion concentration of 34.7 g/L and a divalent tin ion concentration of 29.8 g/L. The proportion of divalent tin ions in the tin ions in this acidic solution was 86% by weight.

Example 7

100 mL of the acidic solution D was charged in the cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, and N₂ gas was blown into the acidic solution at 200 mL/min. Furthermore, 100 mL of 1 N aqueous sulfuric acid solution was charged in the anode electrode chamber. An electrolytic treatment was carried out using an Sn plate of 38×30 mm as a cathode electrode and a platinum plate of 38×30 mm as an anode electrode at a current density of 350 Am⁻² at room temperature (about 25° C.) for 4 hours. Then, the electrolytic treatment was stopped and the content was left to stand for 4 hours to obtain an acidic solution having an indium ion concentration of 287.5 g/L, a total tin ion concentration of 31.1 g/L and a divalent tin ion concentration of 31.0 g/L. The proportion of divalent tin ions in the tin ions in this acidic solution was more than 99% by weight. The impurity concentration in this acidic solution was measured by an ICP spectrometric apparatus to find that concentrations of Al, Si, Fe, Cu and Zn were less than 1 wt ppm and that of Pb was less than 2 wt ppm.

Example 8

100 mL of the acidic solution E was charged in the cathode electrode chamber of an electrolytic cell partitioned with a cation exchange membrane, followed by stirring. In this case, N₂ gas was blown into the cathode chamber at 200 mL/min. Furthermore, 100 mL of 1 N aqueous sulfuric acid solution was charged in the anode electrode chamber. An electrolytic treatment was carried out using an Sn plate of 38×30 mm as a cathode electrode and a platinum plate of 38×30 mm as an anode electrode at a current density of 260 Am⁻² at room temperature (about 25° C.) for 35 minutes. Further electrolytic treatment was carried out at a current density of 310 Am⁻² for 235 minutes. Then, the electrolytic treatment was stopped and the content was left to stand for 4 hours to obtain an acidic solution having an indium ion concentration of 289.6 g/L, a total tin ion concentration of 28.6 g/L and a divalent tin ion concentration of 27.5 g/L. The proportion of divalent tin ions in the tin ions in this acidic solution was 96% by weight. 

1. A method for producing an aqueous solution containing indium ions which comprises a step of subjecting an acidic solution containing indium ions and tin ions to an electrolytic treatment to precipitate metallic tin and a step of removing or redissolving the precipitated metallic tin.
 2. A method according to claim 1, wherein the current quantity for the electrolytic treatment is a quantity with which the tin ions can be substantially precipitated from the acidic solution by converting the tin ions to metallic tin; and the precipitated metallic tin is removed.
 3. A method according to claim 2, wherein the current density in the electrolytic treatment is not less than 50 Am⁻² and not more than 800 Am⁻².
 4. A method according to claim 1, wherein the acidic solution contains indium ions and tetravalent tin ions and the content of the tetravalent tin ions exceeds 50% by weight in the total tin ions; the current quantity for the electrolytic treatment is half or more of the current quantity with which tetravalent tin ions can be precipitated as metallic tin so that the content of divalent tin ions reaches 50% by weight or more in the total tin ions; and the precipitated metallic tin is redissolved.
 5. A method according to claim 4, wherein the current density in the electrolytic treatment is not less than 50 Am⁻² and not more than 2000 Am⁻².
 6. A method according to claim 1, wherein the electrolytic treatment is carried out by passing a current through an aqueous sulfuric acid solution in an anode chamber of an electrolytic cell and an acidic solution containing indium ions and tin ions in a cathode chamber of the electrolytic cell which are partitioned with a cation exchange membrane.
 7. A method according to claim 1, wherein the acidic solution containing indium ions and tin ions is obtained by dissolving a compound containing indium, tin and oxygen in hydrochloric acid. 